Medical tomographic imaging involves capturing radiation transmitted through or emitted from the patient's body in multiple directions, and then processing the captured radiation to reconstruct images of structures within the body, typically in three dimensions. Modern tomographic imaging techniques include, inter alia, computed tomography (CT) based on X-ray transmission and magnetic resonance imaging (MRI), as well as single-photon emission computed tomography (SPECT) using gamma rays, positron emission tomography (PET), and other methods that are known in the art.
In a typical medical tomographic imaging system, the patient lies on a motorized bed (also referred to as a table), which conveys the patient through the bore of the imaging system. The system generally controls the motion of the bed precisely, either automatically or under operator control, in order to position the part of the body that is of interest within the detection volume of the system. In some applications, the bed moves continuously at a controlled speed during imaging, as described, for example, in U.S. Pat. No. 7,738,944.
Magnetic sensing systems are widely used for tracking the position of a probe inside the body of a patient. For example, PCT International Publication WO 1996/05768, whose disclosure is incorporated herein by reference, describes a locating system in which a plurality of field generators produce AC magnetic fields, which are detected by a plurality of sensors at the distal end of an invasive medical instrument. Signals from the sensors are processed in order to find the location and orientation coordinates of the instrument. The CARTO® system, produced by Biosense Webster (Diamond Bar, Calif.), uses this sort of magnetic sensing to track and visualize the location of a catheter inside the patient's body.
Magnetic sensing of catheter position may be used in conjunction with imaging modalities, such as MRI. For example, U.S. Patent Application Publication 2014/0094684, whose disclosure is incorporated herein by reference, describes a medical probe that is suitable for operating in an MRI environment. The probe comprises a flexible insertion tube, which has a distal end for insertion into a body cavity, such as a section of a heart, which is imaged using MRI techniques. A coil in the probe may be used as a position sensor to derive the location and orientation of the distal end of the probe from signals generated when the coil is in an alternating magnetic field having a known spatial distribution. This magnetic field is generated by coils placed at known positions, typically below the patient's torso.